As reported by the Volokh conspiracy, the Pacific Legal Foundation is now asking a Federal court to rule that Obamacare violates the Origination Clause of the Constitution. Article I, Section 7 says:

All Bills for raising Revenue shall originate in the House of Representatives; but the Senate may propose or concur with Amendments as on other Bills.

Since the Supreme Court has ruled that the Individual Mandate is a tax, Obamacare qualifies as a bill for raising revenue; and the text of the bill originated in the Senate, not the House, so the challenge appears to be straightforward.

There's a twist, though: the text of the bill did originate in the Senate, but the actual bill, on paper, originated in the House. The Senate took a bill that had been passed by the House, struck out all of its language, and replaced it in its entirety with the language that was passed by Congress and signed into law by President Obama. So technically, the bill did "originate" in the House; it's just that none of the language that originated in the House actually became part of the final law that was passed.

Does that make the law unconstitutional? At this point, nobody knows. The Supreme Court has ruled in the past that revenue bills which originated in the House but were amended by the Senate were constitutional, but that is explicitly allowed for by the Constitution, as the above quote from Article I, Section 7 makes clear. There has never been a ruling before on a bill in which the entire text was replaced by the Senate. To me, that seems like an obvious attempt to circumvent the constitutional provision, and therefore should not be upheld; but of course I'm not a judge.

It seems inevitable that this issue will reach the Supreme Court one way or the other, since whichever side loses at each lower level of the Federal courts will certainly appeal. It will be interesting to see what the final outcome is, particularly because it looks to me like Chief Justice Roberts is likely to be the swing vote again, just as he was for the Individual Mandate ruling. It will be still more interesting to see how quickly the case is handled, given the upcoming election; it would be extremely quick work to even get it onto the Supreme Court's dockets by November, but the people bringing the challenge are, of course, not unaware of the political implications (to put it mildly). We'll see.